The present invention relates to a liquid crystal display device and a method of manufacturing a liquid crystal display device, and more specifically relates to a liquid crystal display device capable of reducing stress applied to a liquid crystal substance and preventing display defects caused by the stress, and a method of manufacturing the liquid crystal display device.
Along with the recent development of office automation systems, office automation (OA) apparatuses such as word processors, personal computers and PDA (Personal Digital Assistants) have been widely used. With the spread of such OA apparatuses, portable OA apparatuses that can be used in offices as well as outdoors are used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices are widely used as one of the means to satisfy such demands. Liquid crystal display devices not only have small size and light weight, but also have a power saving feature, and are used in television applications in place of a CRT.
A liquid crystal display device comprises a liquid crystal substance sealed in a gap formed by placing two substrates having electrodes so that the electrodes face each other, and applies a voltage across the electrodes to control the light transmittance of the liquid crystal substance which is determined by the applied voltage. A TN (Twisted Nematic) liquid crystal which is used generally has a millisecond-order response speed to the applied voltage, and the response speed is sometimes abruptly decreased to a value near a hundred millisecond value, particularly, in a region with low applied voltage. Consequently, when displaying moving images (for example, 60 images per second) on a liquid crystal display device using a TN liquid crystal, the liquid crystal molecules can not move sufficiently and the images are blurred, and therefore the TN liquid crystal is not suitable for the display of moving images, such as multimedia applications.
Hence, liquid crystal display devices using a ferroelectric liquid crystal (FLC) or an anti-ferroelectric liquid crystal (AFLC) with a spontaneous polarization and a microsecond-order response speed to the applied voltage have been put to practical use. When such a liquid crystal capable of responding at high speed is used for a liquid crystal display device, it is possible to realize an excellent moving image display by controlling a voltage applied to each pixel electrode by a switching element, such as a TFT and an MIM, and completing the polarization of liquid crystal molecules within a short time.
A conventional liquid crystal display irradiates white light of a backlight composed of a discharge light or a light emitting diode from the rear face of a liquid crystal panel, and realizes a color display with color filters provided on the liquid crystal panel. However, if an FLC or an AFLC is used, since the FLC or AFLC has a high speed response, it is possible to perform time-dividing drive (field sequential drive) that realizes a color display by time-dividing lights of the respective light emitters (for example, red, green and blue (primary colors), or cyan, magenta and yellow (complementary colors)). Accordingly, one pixel can display red, green and blue colors, and it is possible, in theory, to realize a three times higher definition display compared to a liquid crystal display device using color filters.
An FLC is known to form a chevron structure, a bookshelf structure or a layer structure composed of a mixture of these structures. If the FLC is used as a liquid crystal substance, there is a drawback that the layer structure is easily broken by stress applied to the liquid crystal substance, such as an external force that changes the gap.
Therefore, in order to maintain the gap of a predetermined distance against an external force, a method in which adhesive columnar spacers are formed between the substrates is used in practical applications. FIG. 1 is a schematic plan view showing a conventional liquid crystal panel. A conventional liquid crystal panel 100 comprises an array substrate 101 and a counter substrate 102 functioning as a pair of insulating substrates made of glass or quartz with good transmittance in a visible light region. The peripheral portions of the array substrate 101 and counter substrate 102 are sealed with a seal member 104 and a closing member 105 through gap maintaining members (for example, columnar spacers) 103 which are provided in a display region 100a to maintain the clearance dimension (gap). The gap formed by sealing is filled with a liquid crystal substance 106 such as an FLC.
Thus, a technique was invented to maintain the gap of a predetermined distance by forming the adhesive columnar spacers 103 between the two substrates to exhibit effects not only against an external force acting in the direction of reducing the gap, but also against an external force acting in the direction of expanding the gap (see, for example, Japanese Patent Application Laid-Open No. 8-110524/1996).
However, if the adhesive columnar spacers 103 are used, the volume of the liquid crystal substance 106 sealed in the gap is determined at the time the liquid crystal is injected, and, when a temperature change occurs, stress is applied to the liquid crystal substance 106 due to the difference between a change in the volume of the liquid crystal substance 106 and a change in the capacity of the space maintained by the columnar spacers 103. If the columnar spacers 103 are formed at high density, cracks or defects 110 (see FIG. 1) due to the stress occur in the peripheral portion (interface 104a) in a predetermined direction because of the differences in the coefficient of linear expansion (hereinafter referred to as the expansion coefficient) and the modulus of elasticity between the display region in which the columnar spacers 103 are provided and the peripheral portion where the seal member 104 is provided, and the defects 110 enter the display region 100a and cause a problem of degradation of display quality.
For example, if a liquid crystal substance showing the phase transition sequence: isotropic phase (Iso phase)—chiral nematic phase (N* phase)—chiral smectic phase (Sc* phase) is used, the uniform alignment state of the Sc* phase is obtained by applying a DC electric field at the time of phase transition from the N* phase to the Sc* phase, and it is presumed that the defects 110 are caused by the difference in the volume shrinkage at the time of phase transition, namely, the difference between the expansion coefficient in the Sc* phase and the expansion coefficient of a panel component member. Note that the defects 110 tend to occur when the columnar spacers 103 are placed at high density, and it is presumed that the degree (the size (length), density, etc. of defects) is determined by not only the difference of the expansion coefficient of the liquid crystal substance 106 from that of the seal member 104, but also the difference of the expansion coefficient of the liquid crystal substance 106 from that of the columnar spacer 103.
Since the expansion coefficient and the modulus of elasticity are physical values varying depending on the environmental temperature, if a temperature change occurs in the environment where the liquid crystal panel 100 is used, the defects 110 may be caused by this change in the same manner as above. For example, when connecting a drive circuit to the liquid crystal panel 100, since a method in which a metal electrode (for example, gold) formed on a FPC and a metal electrode (for example, aluminum) formed on the liquid crystal panel 100 are connected by thermo-compression bonding is used, the defects 110 are caused by the heat. Note that the defects 110 sometimes disappear naturally with the passage of time, but, even when the defects 110 disappeared naturally, there is a possibility that the defects 100 are caused again by a temperature change in the environment where the liquid crystal panel 110 is used.